Tamara V. Mikina

Clathrate formation in water-noble gas (Hydrogen) systems at high pressures

Phase equilibria in helium-water, neon-water, and hxdrogen-water svstems were studied at pressures up to 15 kbar. The results are compared with the data for the previously investigated water systems with argon, crypton, and xenon. It is concluded that classical polyhedral clathrate hydrates are formed in all the systems, the stability of the hydrates diminishing from xenon to neon. In all the systems, except the xenon system, the hydrates are based on the crystalline framework of ice II. Their formation demands high pressures; the larger the guest molecule, the higher the pressure required. The xenon molecule seems to be too large to fit the cage of the ice II framework; therefore, the xenon hydrate CS-I remains stable up to at least 15 kbar.

PHASE DIAGRAM OF THE XE-H2O SYSTEM UP TO 15 KBAR

The phase equilibria in the Xe–H2O system have been studied by the DTA technique under hydrostatic pressures up to 15 000 bar in a temperature range from -25 °C to 100 °C. We have shown that the cubic structure I xenon hydrate forming at ambient pressure does not undergo any phase transitions under the conditions studied. The temperature of its decomposition into water solution and gas (fluid) increases from 27 °C at 25 bar to 78.2 °C at 6150 bar. At higher pressures the hydrate decomposes into water solution and solid xenon. In the temperature range from 6800 to 9500 bar the decomposition temperature (79.0–79.5 °C) is practically independent of pressure, while further pressure increase results in a slow decrease to 67 °C at 15 000 bar.

The formation of solid solutions in the tetrahydrofuran-tetra(n-propyl)ammonium fluoride-water system

The phase diagram of the tetrahydrofuran (THF)-tetra(n-propyl)ammonium fluoride (Prp4 NF)-water system has been studied by differential thermal analysis (DTA). Solid solutions have been detected of the general formula THF·x Prp4NF·(17−2x) H2O, wherex changes from 0.06 to 0.5. The formation of solid solutions is due to the variable degree of the filling of the fragment consisting of four D-cavities in CS-II clathrate hydrates by the tetra(n-propyl)ammonium cation. The solid solution obtained is peculiar in that its vacancies are occupied by the blocks of four D-cavities each. Besides, this kind of displacement of two water molecules by an ionic pair Prp4N++F−, where only the charges of the substituting and substituted groups are equal, takes place here. It appears that this type of solid solution has been discovered for the first time.

Clathrate formation in binary aqueous systems with CH2Cl2, CHCl3 and CCl4 at high pressures

P,T,X phase diagrams of the CH2Cl2-H2O, the CHCl3-H2O and the CCl4-H2) systems have been studied by DTA in the pressure range 10−3 to 5.0 kbar. Under pressure the cubic structure II (CS-II) hydrates forming in all the systems are replaced by hydrates with the composition M·7.3 H2O whose stoichiometry and positive dT/dP values of melting lead us to believe that they are CS-I hydrates.In the CH2Cl2 and CHCl3 systems the nonvariant point coordinates of the hydrate transformationQ2h (l1h17h7l2, where l1 and l2 are liquid phases abundant in water and hydrate former, respectively, h17 and h7 are hydrates with hydrate numbers 17 and 7, respectively) areP = 0.6 kbar, T = −1.5°C andP =2.65 kbar,T = −10.5°C, respectively. In the CCl4 system the 4-phaseQ3h point (l1h17h7s, where ‘s’ is crystalline CCl4) has coordinatesP = 0.75 kbar and T = 0.4°C.The main obstacle of the present study, the very slow achievement of equilibrium, has been eliminated by adding small amounts (0.25% by mass) of surfactants followed by ultrasonic mixing. We have shown that this accelerates the achievement of equilibrium without changing its position.

Clathrate Formation in Tetraisopentylammonium Bromide-Water System

Three polyhydrates of tetraisopentylammonium bromide with 38, 32, and 26 water molecules and also the dihydrate were found in the i-Pent4NBr-H2O system.

Diisoamyldibutylammonium Bromide Clathrate Hydrates

In the system i-Am2Bu2NBr-H2O, along with the known compound i-Am2Bu2NBr·38H2O, three new clathrate hydrates were revealed: i-Am2Bu2NBr·32H2O, i-Am2Bu2NBr·26H2O, and i-Am2Bu2NBr·24H2O. Crystals of all the hydrates were isolated, and their compositions and melting points were determined.

Clathrate Hydrates in the System Tetraisopentylammonium Iodide-Water

Two clathrate hydrates i-Pent4NI·36H2O and i-Pent4NI·32H2O were revealed in the system (i-Pent)4NI-H2O. The hydrates melt incongruently at 14.2 and 14.8°C, respectively. Along with the polyhydrates, tetraisopentylammonium dihydrate was found.